1. Field of the Invention
The present invention relates to a radiation image readout method and apparatus for reading out a radiation image, based on the radiation energy stored on a stimulable phosphor sheet, by projecting an excitation light onto the stimulable phosphor sheet.
2. Description of the Related Art
There are known stimulable phosphors, which upon the irradiation thereof by radiation, store a portion of the radiation energy, and emit upon the subsequent irradiation thereof with a visible light, a laser light, or other excitation light, a stimulated emission corresponding to the stored radiation energy. Radiation image readout apparatuses employing, for example, stimulable phosphor sheets comprising a stimulable phosphor layer containing stimulable phosphors formed on a substrate, to temporarily record the radiation image data of the radiation that has passed through a human body or other subject of photographing upon the irradiation thereof by a radiation, then causing stimulated emission to be generated by irradiating the stimulable phosphor sheet with an excitation light such as laser light, are in wide use as CR (Computed Radiography) apparatuses. Further, in accordance with the radiation image readout apparatus utilizing the stimulable phosphor sheet described above and with a view to reduce the readout time of the stimulated emission as well as making the apparatus more compact and of reduced cost, configurations have been proposed wherein: a line light source is used as an excitation light source for projecting an excitation light in a line beam onto the stimulable phosphor sheet, together with a line sensor formed of a plurality of photoelectric converting elements arranged in a straight line along the lengthwise direction of the line-shaped portions of a stimulable phosphor sheet that have been irradiated with the excitation light emitted from the line light source (hereinafter referred to as the main scanning direction); and a scanning means moves the line light source and line sensor relative to the stimulable phosphor sheet from one end of thereof to the other, in the direction substantially perpendicular to the lengthwise direction of the aforementioned line-shaped portions (hereinafter referred to as the sub-scanning direction). Refer to, for example, Japanese Unexamined Patent Publication Nos. 60 (1985)-111568, 60(1985)-236354, and 1(1989)-101540.
Further, there are widely known, as described in Japanese Unexamined Patent Publication Nos. 1(1989)-60784, 1(1989)-60782 and 4(1992)-3952, autoradiography detection systems employing stimulable phosphor sheets, wherein: a material that has been radioactively labeled is administered to an organism; the organism or a part of the organism is taken as a sample and overlaid on a stimulable phosphor sheet for a predetermined time interval so as to cumulatively record the radiation energy of the radioactively labeled sample onto the stimulable phosphor sheet; an excitation light beam such as a laser beam is scanned over the stimulable phosphor sheet, thereby causing each part of the stimulable phosphor sheet exposed to the excitation light beam to emit a stimulated emission; and the stimulated emission is photoelectrically detected, whereby an image signal representing the radiation image of the sample is obtained. According to the autoradiography detection systems, the excitation light is caused to scan over the entire surface of the sample by moving the optical system in both the main scanning direction and the sub-scanning direction relative to a stage on which the sample has been placed and maintained in a stationary position, or by moving the optical system in the main scanning direction, in which the excitation light is required to scan the sample at a high speed, and moving the stage in the sub-scanning direction, in which the excitation light can scan the sample at a relatively low speed.
The image signal obtained in each of the aforesaid systems is subjected to image processing such as gradation processing, frequency processing and/or the like appropriate for rendering the image to be read, and a radiation image of the sample is reproduced, based on the processed radiation image signal, on a recording medium such as a photographic film or a display such as a high-resolution CRT as a visible diagnostic image (a final image) for diagnosis. The visible diagnostic image is then diagnostically read by a physician, or subjected to a quantitative analysis by a computer to perform the diagnosis.
Further, there have been developed microarray detection systems, wherein: known binding materials, e.g., hormones, tumor markers, enzymes, antibodies, antigens, abzymes, other proteins, nucleic acids, cDNAs, DNAs, mRNAs and the like, each of which is capable of binding to a specific organism-derived material, according to known properties such as the sequence, lengths, the composition and/or the like of bases, are applied in droplets, by use of a spotting apparatus, onto a substrate such as a membrane filter to form a microarray of independent spots; an organism-derived material, e.g., hormone, tumor marker, enzyme, antibody, antigen, abzyme, other protein, nucleic acid, cDNA, DNA, mRNA or the like, which has been obtained as a sample from an organism by simple separation, an extraction process or the like and chemically processed or modified as desired, and radioactively labeled, is hybridized with the known binding materials on the microarray, the binding materials to which the sample material binds are radioactively labeled; the microarray is brought into close contact with a stimulable phosphor sheet to expose the stimulable phosphor sheet in the pattern of distribution of the radioactive labeling on the microarray; an excitation light beam such as a laser beam is scanned over the stimulable phosphor sheet, thereby causing the stimulable phosphor sheet exposed to the excitation light beam to emit a stimulated emission; and the stimulated emission is photoelectrically detected, whereby an analysis of the organism-derived material sampled from the organic body is obtained. According to the microarray detection system, as in the autoradiography detection system, the excitation light scanning system can be of a configuration wherein: the excitation light beam is scanned over the entire surface of the sample by moving the optical system in both the main scanning direction and the sub-scanning direction with respect to a stage on which the substrate has placed held stationary, or by moving the optical system in the main scanning direction, in which the excitation light is required to scan the sample at a high speed, and moving the stage in the sub-scanning direction, in which the excitation light can scan the sample at a relatively low speed. In the case of the microarray detection system, since the sample is small in size, the excitation light beam may be caused to scan the surface of the support by moving the stage in the main scanning direction or in both the main scanning direction and the sub-scanning direction.
According to the above-described systems wherein a stimulable phosphor sheet, on which radiation energy has been cumulatively stored, is scanned in the main scanning direction and in the sub-scanning direction by an excitation light beam, and the stimulated emission emitted from the stimulable phosphor sheet upon the irradiation thereof by the excitation light beam is detected, because there are cases in which it is necessary to read out at a high resolution the radiation image stored on the stimulable phosphor sheet, in order to prevent the scattering of the excitation light within the interior portion of the stimulable phosphor sheet in such cases, a colorant, such as an navy blue colorant that selectively absorbs the excitation light can be added to the stimulable phosphor sheet in advance, whereby the diffusion rate of the excitation light can be controlled, and the readout can be performed at a high resolution.
However, if a stimulable phosphor sheet contains a colorant such as a navy blue colorant, because excitation light and the light of the stimulated emission is absorbed by the colorant, the sensitivity thereof is deteriorated.
The present invention has been developed in view of the forgoing problems, and it is an objective of the present invention to provide a radiation image readout method and apparatus for scanning an excitation light in a main scanning direction and a sub scanning direction over a stimulable phosphor sheet, on which radiation energy has been cumulatively stored, and detecting the stimulated emission emitted from the stimulable phosphor sheet upon the irradiation thereof by the excitation light; wherein the radiation image stored on the stimulable phosphor sheet can be read out at a high resolution and high sensitivity.
The radiation image readout method according to the present invention comprises the steps of: scanning an excitation light in a main scanning direction and a sub scanning direction over the surface of a stimulable phosphor sheet, on which radiation energy has been cumulatively stored, so as to two-dimensionally irradiate the stimulable phosphor sheet with the excitation light; receiving and photoelectrically converting the stimulated emission emitted from the stimulable phosphor sheet upon the irradiation thereof by the excitation light to obtain an output signal, thereby performing readout; wherein, when the readout of a plurality of stimulable phosphor sheets having different diffusion rates with respect to the excitation light is to be performed by a single apparatus, the readout is performed by emitting an excitation light of an energy level which is greater as the excitation light diffusion rate of the stimulable phosphor sheet is smaller.
Here, because the intensity of the stimulated emission emitted from the stimulable phosphor sheet is proportional to the intensity of the excitation light with which the stimulated phosphor sheet has been irradiated, although it is thought that the energy of the excitation light should be made large in order to perform a high sensitivity detection, in actuality, even if the energy level of the excitation light were made large, if the magnitude of the energy level should exceed a predetermined size, the sensitivity is not improved; in fact it deteriorates. It has been clearly determined by an experiment conducted by the inventors of the present invention that this is due to the fact that overlap is caused to occur with the adjacent scanning lines (the next line to be scanned or the previously scanned line) owing to the scattering of the excitation light within the stimulable phosphor layer, whereby the stimulable phosphors of the adjacent scanning lines are also stimulated due to the scattered excitation light; when an actual readout is to be performed, a so-called empty reading (the excitation light is scanned over positions of which the stored radiation energy has already been dissipated by the energy of the scattered excitation light) is produced.
Further, because dissipation of the stored radiation energy caused by the scattered excitation light exerts a significant influence in the sensitivity reduction of the stimulable phosphor sheet, by comparing the excitation light energy level Ib, which is the energy level at which deterioration of the sensitivity of the stimulable phosphor sheet begins in a stimulable phosphor sheet containing an navy blue colorant by which the diffusion of the excitation light is controlled, and the excitation light energy level Iw, which is the energy level at which deterioration of the sensitivity of the stimulable phosphor sheet begins in a white stimulable phosphor sheet not containing an navy blue colorant, as shown in FIG. 2 (the relation between the stimulated emission output and the excitation light energy level for a stimulable phosphor sheet containing the navy blue colorant is shown by the solid line 1, and the relation between the stimulated emission output and the excitation light energy level for a white stimulable phosphor sheet not containing the navy blue colorant is shown by the solid line 2), it has been clearly shown by the experiment of the inventors of the present invention that the excitation light energy level Ib at which the deterioration of the sensitivity of the stimulable phosphor sheet containing the navy blue colorant begins to be caused is larger than the excitation light energy level Iw.
Therefore, according to the radiation image readout method of the present invention, when a single apparatus is to be used to read out stimulable phosphor sheets containing the navy blue colorant and white stimulable phosphor sheets not containing the navy blue colorant, that is to say, when stimulable phosphor sheets having different excitation light diffusion rates are to be read out, the excitation light is to be emitting an excitation light of an energy level which is greater as the excitation light diffusion rate of the stimulable phosphor sheet to be read out is smaller. Further, it is desirable that the excitation light energy level be controlled so as to be of the level indicated by Ib, Iw shown in FIG. 2.
Further, the referents of xe2x80x9cscanning an excitation light in a main scanning direction and a sub scanning direction over the surface of a stimulable phosphor sheet, on which radiation energy has been cumulatively stored, so as to two-dimensionally irradiate the stimulable phosphor sheet with the excitation lightxe2x80x9d can include, more specifically, any method of two-dimensionally irradiating the stimulable phosphor sheet with the excitation light: For example, a spot beam excitation light can be two-dimensionally, that is, in the main scanning direction and the sub-scanning direction, scanned over a stimulable phosphor sheet that is maintained in a fixed position; the stimulable phosphor sheet can be moved in either the main scanning direction or the sub-scanning direction, and the excitation light can be scanned over the sheet in the other direction to two-dimensionally irradiate the sheet with the excitation light; or the stimulable phosphor sheet can be moved relative to the excitation light in both the main scanning direction and the sub-scanning direction. Further, a line light source or the like can be used to scan the sheet in the main scanning direction with a line beam excitation light, and the line light source or the sheet can be moved in the sub-scanning direction to two-dimensionally irradiate the sheet with the excitation light.
Still further, the referent of xe2x80x9cexcitation light energyxe2x80x9d is the quantity of energy of the excitation light per unit of surface area of the phosphor sheet onto which the excitation light is projected. The magnitude of the the excitation light energy can be controlled by, for example, by controlling the intensity of the excitation light, or the scanning speed of the excitation light in at least one of the main scanning direction and the sub-scanning direction.
The radiation image readout apparatus according to the present invention comprises: an illuminating means for two-dimensionally scanning, by scanning in a main scanning direction and a sub-scanning direction, a line beam excitation light over the surface of a stimulable phosphor sheet on which a radiation image has been recorded; and a photoelectrical converting means for receiving and photoelectrically converting the stimulated emission emitted from the portions of the stimulable phosphor sheet which have been irradiated by the excitation light; further comprising a recognizing means for discerning the data relating to the excitation light diffusion rates of a plurality of types of stimulable phosphor sheets; and an excitation light energy controlling means for controlling, based on the data related to the stimulable phosphor sheet excitation light diffusion rate recognized by the recognizing means, the energy level of the excitation light so that the excitation light is emitting an excitation light of an energy level which is greater as the excitation light diffusion rate of the stimulable phosphor sheet to be read out is smaller.
Here, the xe2x80x9cphotoelectrical converting meansxe2x80x9d can be any means that converts the light of the stimulated emission to electric signals, e.g., a photomultiplier, a CCD sensor, or a line sensor having a plurality of photoelectrical converting elements arranged in a straight line.
Further, the referents of xe2x80x9cdata relating to the excitation light diffusion ratexe2x80x9d can include any data that represents the diffusion rate, e.g., data recorded on a bar code attached to the stimulable phosphor sheet, or data inputted from a predetermined input means; said data can be the diffusion rate itself of the phosphor sheet to be read out, or data related to the type of the stimulable phosphor sheet.
Still further, when the plurality of stimulable phosphor sheets consists of stimulable phosphor sheets containing a predetermined colorant for suppressing the diffusion rate and white stimulable phosphor sheets not containing the predetermined colorant, the excitation light energy control means controls the excitation light energy so that the excitation light projected onto the stimulable phosphor sheet containing the predetermined colorant is of a greater energy level than that of the excitation light projected onto the white stimulable phosphor sheet.
Here, the referents of xe2x80x9cpredetermined colorantxe2x80x9d include colorants that selectively absorb excitation light; it is desirable that the colorant be of a reflection rate wherein the average reflection rate of the light in the excitation light wavelength range is sufficiently smaller than the average reflection rate of the wavelength range of the stimulated emission emitted by the stimulable phosphors(s) employed in the stimulable phosphor sheet. The colorant can be a pigment such as a navy blue, cobalt blue, cerulean blue, oxidized chromium, TiO2xe2x80x94ZnOxe2x80x94CoOxe2x80x94NiO, and the like.
Further, the excitation light energy projected onto a stimulable phosphor sheet containing a predetermined colorant can be 1.5 or more times the excitation light energy projected onto the white stimulable phosphor sheet.
Still further, the excitation light energy projected onto a stimulable phosphor sheet containing a predetermined colorant can be 3.0 or more times the excitation light energy projected onto the white stimulable phosphor sheet.
In addition, the excitation energy control means can be a means for further controlling the excitation light energy according to the pitch of the sub-scanning direction of the portions of the stimulable phosphor sheet irradiated with the line beam excitation light in the main scanning direction.
Here, the expression xe2x80x9ccontrolling the excitation light energy according to the pitch of the sub-scanning directionxe2x80x9d refers to, for example, controlling the excitation light energy so that the relation between the excitation light and the output signal for the pitch of the sub-scanning direction energy, due to the effect whereby the radiation energy on adjacent scanning lines is dissipated by the scattered excitation light, is that shown in FIG. 3 (for cases in which the sub-scanning direction pitch is 25 xcexcm, 50 xcexcm, and 100 xcexcm, respectively); wherein, on the one hand, the magnitude of the output signal increases as the magnitude of the excitation light energy increases up to a predetermined magnitude of excitation light energy, and on the other hand, the magnitude of the output signal decreases due to the effect of the dissipation of the radiation energy stored on adjacent scanning lines if the excitation light energy exceeds the predetermined value, for any of the above-described pitches. And, the magnitude of excitation light energy at which the deterioration of the magnitude of the output signal is initiated becomes smaller in proportion to the smallness of the pitch. Accordingly, the excitation light energy is controlled so that an adequate signal level is obtained at each pitch; for example, I25, I50, I100, wherein the excitation energy is controlled to the level whereat the largest output signal for each respective pitch is obtained.
Further, the pitch in the sub-scanning direction of the portions to be scanned with a line beam excitation in the main scanning direction can be less than or equal to 50 xcexcm, or even less than or equal to 25 xcexcm.
According to the radiation image readout method and apparatus of the present invention, when the readout of a plurality of stimulable phosphor sheets having different excitation light diffusion rates is to be performed by a single apparatus, because the readout is performed by emitting an excitation light of an energy level which is greater as the excitation light diffusion rate of the stimulable phosphor sheet is smaller, even for a stimulable phosphor sheet having a small excitation light diffusion rate with respect to the excitation light energy required for a high resolution readout thereof, the readout can be performed at a high sensitivity.
Further, when the plurality of stimulable phosphor sheets consists of stimulable phosphor sheets containing a predetermined colorant for suppressing the diffusion rate of the excitation light and white stimulable phosphor sheets not containing the predetermined colorant, for cases in which the excitation light energy is controlled so that the excitation light projected onto the stimulable phosphor sheets containing the predetermined colorant is of a greater energy level than that of the excitation light projected onto the white stimulable phosphor sheets, the same effect as described above, wherein a stimulable phosphor sheet requiring a high-resolution readout can be read out at a high sensitivity, can be obtained for stimulable phosphor sheets containing a predetermined colorant and white stimulable phosphor sheets not containing the predetermined colorant, respectively. At this time, for cases in which navy blue is used as the predetermined colorant, if the readout is performed at the respective excitation light energy levels Ib, Iw as shown in FIG. 2, the readout can be performed at an even higher sensitivity.
Still further, for cases in which the excitation energy control means is a means for further controlling the excitation light energy in accordance to the pitch of the sub-scanning direction of the portions of the stimulable phosphor sheet irradiated with the line beam excitation light in the main scanning direction, the deterioration of the sensitivity of the stimulable phosphor sheet due to the dissipation of the radiation energy stored on adjacent scanning lines caused by the scattering of the excitation light within the stimulable phosphor layer can be reduced, and the readout can be performed at a high sensitivity and a high resolution.
In addition, for cases in which a high-resolution readout has been performed wherein the pitch in the sub-scanning direction is less than or equal to 50 xcexcm, or even less than or equal to 25 xcexcm, because the excitation light energy level is controlled in accordance to the pitch, the effect whereby the sensitivity of the stimulable phosphor sheet is deteriorated due to the dissipation of the radiation energy stored on adjacent scanning lines caused by the scattering of the excitation light within the stimulable phosphor layer can be reduced, and the readout can be performed at an even higher sensitivity.